The air conditioning systems that have hitherto been used most widely for performing both cooling and heating operations are those which include a cooling apparatus having a built-in electric heater or those which include a cooling apparatus having a built-in hot water coil. However, the former require high operation cost when the system performs a heating operation, while the latter, which depend on changes in water temperature or in sensible heat for the transfer of heat energy, require a high rate of heating medium circulation for their heating capacity. Consequently they need circulating pumps with large power consumption and large-diameter piping for the circulation of the heating medium. Another disadvantage is the cumbersomeness of pipework involved.
On the other hand, cooling and heating systems of the so-called heat pump type which utilize the ordinary refrigeration cycle are in use as installations taking advantage of changes in phase of the heating medium to receive and supply heat, thus carrying out the circulation of the medium at a low flow rate. The heat pump type equipment, which uses outdoor air as the heat source, has a limitation in that its heating capacity decreases as the outdoor air temperature drops. Since the tendency is that the lower the outdoor air temperature the greater the heating load requirement, the heat pump system offers a fatal disadvantage of insufficient heating capacity when it is very cold outdoors.
Moreover, since the conventional heat pump installations utilize outdoor air as the heat source and draw in the low-pressure refrigerant evaporated at a temperature level below the outdoor air temperature, the specific volume of the refrigerant vapor thus taken into a compressor that serves as the circulating pump is too large and the delivery is too small as compared with the cylinder volume. These shortcomings combine with a large pressure difference that occurs between the entrance and exit of the compressor to increase the power consumption of the compressor unduly for its heating capacity.